We investigated extraordinary patterns of sex allocation in captive eclectus parrots (Eclectus roratus). These birds are extremely unusual as they show reverse sexual dichromatism, they are the only cooperatively breeding parrot, and they are one of the few birds with nestlings that are easily sexed. They lay two eggs per clutch, but often only fledge one young, and the sex ratio of 209 fledglings did not differ significantly from parity. However, when two young are fledged together they are very likely to be of the same sex, and some females produce long unbroken runs of one sex (the maximum was 20 males) before switching to the other sex. Monte–Carlo simulations show that these runs of same–sex clutches defy expectation if we assume that the sex of chicks within each clutch is independent of the previous clutch. We use further simulations to show that the sex bias must occur at fertilization (i.e. the primary sex ratio), although the female may make further adjustments via infanticide. Control over sex allocation in eclectus parrots is one of the most extreme reported from birds.